NMR Spectroscopy of Cyclodextrin–Inorganic Anion Systems

Abstract

Abstract The addition of alkali salts of chaotropic anions such as Br−, I−, SCN−, N3−, ClO4−, and NO3− to an α-cyclodextrin (α-CD) solution in D2O caused marked downfield shifts in the 1H NMR signal of the C(5)–H located in the interior of the α-CD cavity, whereas the signal showed no or only small upfield shifts upon the addition of alkali salts of antichaotropic anions such as F−, HCO3−, H2PO4−, HPO42−, and SO42−. The effects of the inorganic salts on the chemical shifts (δ) for the other protons of α-CD were significant but smaller than that on δ for C(5)–H. These facts indicate that the chaotropic anions are included within the α-CD cavity and held in the vicinity of C(5)–H. Similar, but smaller, effects of the inorganic salts were observed for β- and γ-CD. The binding constants and thermodynamic parameters determined for complexation of CD’s with some chaotropic anions suggest that hydrophobic and van der Waals interactions, as well as a conformational change of CD macrocycles and the desolvation of CD’s and/or the anions upon complexation, play important roles in complex stabilization. The effect of inorganic salts on the spin-lattice relaxation times of α-CD protons also supports the above conclusion.